Abstract: Examine the effect of including cell-phone numbers in a traditional landline random digit dial (RDD) telephone survey.The 2007 California Health Interview Survey (CHIS).CHIS 2007 is an RDD telephone survey supplementing a landline sample in California with a sample of cell-only (CO) adults.We examined the degree of bias due to exclusion of CO populations and compared a series of demographic and health-related characteristics by telephone usage.When adjusted for noncoverage in the landline sample through weighting, the potential noncoverage bias due to excluding CO adults in landline telephone surveys is diminished. Both CO adults and adults who have both landline and cell phones but mostly use cell phones appear different from other telephone usage groups. Controlling for demographic differences did not attenuate the significant distinctiveness of cell-mostly adults.While careful weighting can mitigate noncoverage bias in landline telephone surveys, the rapid growth of cell-phone population and their distinctive characteristics suggest it is important to include a cell-phone sample. Moreover, the threat of noncoverage bias in telephone health survey estimates could mislead policy makers with possibly serious consequences for their ability to address important health policy issues.